The present invention relates generally to a field of sensors, and, more particularly, to designs and methods of assembly of tactile array sensors used for sensing pressure distribution exerted at various points of a flexible and pliable object. Specifically, the present invention describes the use of a flex harness containing a plurality of interconnect electrodes allowing convenient and repeatable manufacturing of tactile sensors with high number of closely located conductors.
Tactile sensing involves a continuous, variable measuring of tactile force or pressure. In some respects, tactile sensing for electromechanical devices is analogous to the human sense of touch in that information about the amount and distribution of tactile pressure over a surface can be received and transmitted. Not surprisingly, tactile sensing finds great utility in the field of robotics where the tactile sensors provide signals for negative feedback control of servomechanisms and the like. Tactile sensing can provide information about shape, texture, position, orientation, deformation, center of mass, and presence of torque or slippage with respect to an object in contact with the sensor. Other applications of tactile sensing will come to mind to those skilled in the art.
The tactile sensor or tactile sensing transducers can be configured with an array of electrodes to provide a measure of the distribution of tactile pressure over a surface. Ideally, the tactile sensor will have sufficient sensitivity, consistent reproducibility, and high resolution.
One of the known methods employed in tactile sensing is the use of a medium whose electrical properties vary in response to pressure induced deformation. For example, some materials exhibit a piezoresistive effect, i.e. the electrical resistance of the material varies in response to its deformation. Layers of such material sandwiched between two conductive plates will provide a means for detecting pressure when an electrical potential is established between the two plates. The current flowing between them will therefore vary according to the deformation of the intermediate layer resulting from an external pressure forcing the plates closer together. The current will change according to Ohm's Law, such that measuring the current can provide a means for measuring the tactile force applied to the plates. Compression sensitive materials currently in use include for example foamed polymers, which contain conductive fillers such as finely divided particles of metal or carbon. Polyurethane and silicone are also commonly used.
There are also known bi-dimensional capacitive pressure sensors developed primarily for realizing the so called “touch pads” of portable PCs that allow the reconstruction of the position of an object weighing onto the surface of the sensor. U.S. Pat. No. 5,374,787 describes a sensor of the position of such an object onto a sensible surface. These devices are realized with manufacturing techniques of printed circuit boards (PCB), according to which a substrate of fiber-glass or of Mylar® is provided with copper orthogonal stripes defined on one or on the other face of the substrate. Notwithstanding that a substrate of Mylar® or of another dielectric material may be moderately flexible, at least for small deflections, the sensor so constructed remains substantially rigid and not pliable into different geometric shapes. It is evident that these known devices are unsuitable for covering multi-curvature shapes such as a robot fingertip, other organic shapes, or to be incorporated in any object that must retain flexibility and pliability to conform to different shapes as a fabric.
FIG. 1 illustrates the principle of creating a typical cloth-based tactile sensor array. Such device generally consists of a top plurality of parallel electrodes 10 that are placed over the bottom plurality of electrodes 20 with a non-conductive elastic isolation layer therebetween. Both the top 10 and bottom 20 plurality of electrodes can be made of a cloth-based material such as LYCRA™ that can be stretched in one or both X and Y directions. Other materials such as weave fabrics can also be used for this purpose. Individual electrodes can be made as metallized fibers, strands, or yarns that form such cloth or in any other way that is known from the prior art. If soldering is to be used to connect electrodes to the wires of the control unit, the temperature stability of the fabric material should be chosen to allow soldering to take place. In a typical configuration shown on FIG. 1, top electrodes are positioned to be perpendicular to the bottom electrodes forming the intersection areas, which define individual pressure sensors.
The non-conductive material separating the two layers with electrodes is typically chosen to be elastic and compress under the force applied to it within the range of forces estimated to be the working range for each tactile sensor array. For each of the intersection areas in which the top electrode intersects the bottom electrode, a capacitor is therefore formed between the top electrode and the bottom electrode with a compressible non-conductive material therebetween. That capacitor is used as an individual pressure sensor. As the pressure of force is applied to each such sensor, the top electrode is moved closer to the bottom electrode with the compression of the non-conductive material separating the two electrodes. Voltage potential is applied to both the top electrode and the bottom electrode so the capacitance can be measured therebetween. Changing capacitance reflects the degree of pressure or force applied to each sensor in the array. Typically, one plurality of electrodes is designated as a Drive Strip and the other plurality is designated as a Sense Strip. Drive electronics can provide selective measuring of capacitance at any chosen point between these strips of electrical conductors. High speed scanning of all the points in the matrix results in a single data frame reflecting pressure distribution over the surface of the matrix.
As discussed above, flexible tactile sensor arrays can provide useful information about pressure distribution along curved surfaces. Despite the great extent of knowledge developed in the prior art, practical use of tactile cloth-based sensor arrays has been limited until the present time. This is caused by the difficulties in manufacturing the tactile sensor array with more than just a few electrodes. Once the number of electrodes exceeds about 8 on each side of the array (the total of 16), direct attachment of the control unit wires to cloth-based electrodes becomes a burdensome procedure. One great difficulty is managing the large number of wires on both sides of the tactile sensor array and connecting them repeatedly in a reliable manner without intermittent opens and shorts between wires or electrodes.
Another manufacturing difficulty associated with the prior art tactile sensors stems from the high pitch density sensor designs having electrodes located closely together, typically less than only about 3 mm apart. Attaching individual wires to such electrodes becomes difficult as conductive epoxy or solder exhibits a tendency during assembly to deploy over more than one interconnection pad and therefore short the electrical connections.
Various means of terminating the cloth conductors have been described in the prior art. The U.S. Pat. No. 6,826,968 by Manaresi describes a textile-type pressure array sensor adapted to be used on a pliable surface of a flexible object such as a sail. It describes the basic matrix structure of such type of arrays and teaches soldering as a way of terminating the electrodes of the sensor array matrix to a respective pad of the PCB board.
PCT Application No. WO 01/75778 by Swallow et al. describes a pressure-sensitive textile fabric in which the electrodes are incorporated into the flexible fabric itself. Electrode termination is achieved by the use of electrical bus-burs that “may be sewn, embroidered, printed, adhered, mechanically clamped or crimped to the piece of fabric in order to make an electrical contact with the matrix of conductors”. The disadvantage of this arrangement is that the rigid bus-bur attached to the edge of the tactile sensor will make this edge itself rigid, loosing the original advantages of the cloth flexibility.
PCT Application No. WO 01/75924 by Sandbach et al. describes a detector made from electrically conductive fabric. Termination of conductive tracks is done with the use of a printing process with electrically conductive ink, or attaching the conductive elements by a conductive adhesive. Although this simplified approach maybe fine for rigid and semi-rigid flex circuit technology, it isn't a reliable solution for pressure array sensors that are flexible due to the substrate flexing under load and damaging the electrically conductive ink or adhesive.
Kim describes a method and apparatus for sensing the tactile forces in his U.S. Pat. No. 4,555,954. The specification mentions the use of a pair of clamping members 26 and 28 to connect the entire electrode strip 18 to the circuit board 12. Again, this termination method makes for a rigid edge of the otherwise flexible tactile sensor.
Japanese Patent No. 2002-203996 describes the pressure array matrix and illustrates the method of electrode termination on FIGS. 3(a) and 3(b). The exact attachment method is not clear from these drawings but appears to use direct wire connection methods described by others.
Wymore discloses a tactile tracking system in his U.S. Pat. No. 6,515,586. Data bus lines and the transmitter 308 are clamped onto the carpet via the use of low profile crimp-on connectors. This conceptual patent fails to provide design specifics needed for creating of a practical device.
Burgess describes a tactile sensing transducer in his U.S. Pat. No. 5,060,527. The silk screen technique is described to illustrate the process of creating electrode lines, that are either bonded using a conductive adhesive or attached to it via pressure from an electrical contact. Disadvantages of these approaches include low reliability and durability in the case of conductive adhesive or the need for pressure from a rigid substrate that holds the electrical contacts to exert the necessary pressure.
Prior art designs have several common disadvantages such as low reliability, bulkiness and/or fabrication difficulties. The need exists for a design allowing simple, reliable and repeatable high volume manufacturing of tactile array sensors having a substantial number of conductors. The need also exists for a design of a tactile array sensor allowing simple and reliable manufacturing of tactile sensors with closely located conductors.